Electrode substrate for electrochemical cells based on low-cost manufacturing processes

ABSTRACT

This invention relates to electrode substrates for electrochemical cells, particularly low-temperature fuel cells, and processes for their production. Graphitized fiber web structures are used that have a preferred non-planar fiber alignment resulting in high through-plane conductivity. These structures are further impregnated and processed to adjust the final product properties.

This application claims the benefit of provisional application No.60/142,702 filed Jul. 7, 1999.

FIELD OF THE INVENTION

This invention relates to electrode substrates for electrochemicalcells, particularly polymer electrolyte membrane fuel cells (PEMFC) andPhosphoric Acid Fuel Cells (PAFC), and processes for their production.

BACKGROUND OF THE INVENTION

A fuel cell converts fuel, such as hydrogen, and an oxidant, typicallyoxygen, to electricity and reaction products. This electrochemicalreaction is facilitated by electrocatalysts, typically from the platinumgroup.

Fuel cells typically are constituted of units, as shown in FIG. 1,called single cells 1, comprising an electrode assembly 1′ where amembrane or electrolyte layer 2 is sandwiched between two electrodes 3and 4, individually referred to as anode 3 and cathode 4. Theseelectrodes are typically flat and have at least two parallel surfaces,the membrane or electrolyte layer 2 being positioned between thesesurfaces of the two electrodes.

Each of the electrodes 3 and 4 is composed of a porous conductiveelectrode substrate 3′ and 4′, usually made of carbon fiber paper orcarbon cloth, and a thin electrocatalyst layer 3″ and 4″, preferablycomprising finely divided platinum or other noble metal catalysts.

When using hydrogen as fuel, the fuel gas is oxidised at the anode 3yielding protons and electrons. The former migrate through the membranelayer 2 from the anode to the cathode 4, while the electrons aretransported through an external circuit to the cathode 4. At the cathode4, oxygen is reduced by consumption of two electrons per atom, to formoxide anions which enter the electrolyte layer and react with theprotons that have crossed the electrolyte layer to form water. As shownin this FIG. 1, separator plates 5 and 6 which are adjacent to theelectrodes 3 and 4, may incorporate grooves 8 and 9 on the surfacesopposite to the electrodes providing access for the fuel and oxidant tothe electrodes. The separator plates 5 and 6 can be covered with currentcollector plates 7 and 7′ usually made of metal which also act asconductive connection between two adjacent single cells.

PEMFC generally employ a membrane electrode assembly (MEA, 1′) as singlecell comprising a thin polymer membrane 2 with high proton conductivityplaced between two electrode sheets 3 and 4. PAFC single cells aretypically constituted of a thin phosphoric acid containing matrix layer2 sandwiched between the two electrodes 3 and 4.

The electrodes 3 and 4 mainly comprise of an electrically conductive andchemically inert electrode substrate (ES) 3′ and 4′ and anelectrocatalyst layer (3″ and 4″) facing the membrane or electrolyte 2.The ES has a porous structure to provide an efficient entry passage andplanar distribution for the fuel and oxidant to the catalyst layers 3″and 4″ as well as an exit for the reaction products away from thecatalyst layer. It also features other important properties such as highelectrical conductivity, chemical stability, mechanical strength, andhomogeneity.

As is shown in FIG. 1, it is advantageous to separate the functions ofproviding access and distributing fuel and oxidant (established by thegrooves 8 and 9 in the separator or distributor plates 5 and 6 inFIG. 1) and the support of the catalyst layer 3″ and 4″ by the electrodesubstrates 3′ and 4′. The separator or distributor plates 5 and 6 areusually made of metal or other conductive materials as they shall alsoserve to collect the current. They incorporate grooves 8 and 9 or othermeans of distribution of liquids or gases. These separator plates arestacked on the electrode substrates on the side opposite the electrolytelayer 2.

Current can be collected in the distributor or separator plates (asmentioned above), or in separate current collector plates which can be asolid metal sheet if they form the outer part of the assembly, or can bea mesh or porous conductive plate if they are stacked between the fuelfeed and the electrodes (between 4 and 6, or between 3 and 5, in anassembly as otherwise shown in FIG. 1). It is also possible to combinethe separator plates and current collector plates.

Since various gases and liquids have to permeate through the ES, highporosity is a preferable feature of an ES. At the same time, the poresize distribution needs to be adjusted to the general characteristics ofpractical fuel cells. The grooves in the electrode substrates provide avery coarse distribution of fuel and oxidant. These need to be evenlytransported and finely distributed to the catalyst layer through the ES.Furthermore, various types of gases and liquids have to be transportedthrough the ES which requires fine-tuning and adaptation of the ESporous network. Hence, adjusting the degree of porosity as well as poresize and its distribution of an ES is important for the performance of afuel cell.

Equally important is the through-plane (perpendicular to the largesurface) electrical conductivity of the ES since they provide aconductive path between the catalyst layer and the separator or currentcollector plates. A low electrical conductivity can result insubstantial power losses of the fuel cell. Usually, high porosity of anES has to be balanced against improved through-plane conductivity orvice versa.

Mechanical properties of ES play an increasingly important role for theproduction of commercial fuel cells since the ES are being handled byautomatic equipment, and product integrity determines the commercialsuccess of fuel cells.

In the light of fuel cell commercialisation efforts, ES are alsorequired to be processable as a continuous roll material. This allowsthe application of industrial scale processes for the catalyst layerdeposition and other required manufacturing steps.

Furthermore, a continuous roll ES provides high homogeneity and productuniformity in comparison with ES produced in a batch-mode.

Commonly used ES materials for fuel cells include carbon fibers (papers,felt, and woven cloth), metal fibers (mesh or gauze), and polymers(gauze filled with carbon materials).

A carbon fiber paper ES is usually made in such way that the carbonfibers are aligned mainly in planar direction. Due to the highanisotropy of carbon fibers, the in-plane conductivity of such carbonfiber paper is high but through-plane conductivity is poor. Such carbonfiber paper can be rendered suitable as ES for fuel cells if it ismanufactured using a carbonisable binder followed by carbonising thisproduct at high temperatures to achieve satisfactory through-planeconductivity (cf. U.S. Pat. No. 4,851,304). This type of ES is shown asa cross-section in FIG. 2. Carbon fibers 10 are aligned mainly in planardirection; carbonised binder particles 11 contribute to the mechanicalstability of the ES. Carbonisable binder in this context means a binder,usually a binder resin which cross-links under the action of heat, thatcan be converted to elemental carbon in a high yield when heated for aprolonged time, i. e. more than 5 minutes up to several hours, above thedecomposition temperature with the exclusion of oxygen or oxidisinggases. This expensive batch-process yields ES with poor mechanicalproperties. WO 98/27606 relates to a process for filling carbon fiberpapers and polymer substrates having low through-plane conductivity withconductive materials. The ES resulting from this procedure still lack ahigh through-plane conductivity and have a low porosity because thepores of the starting materials have to be filled with a high fractionof conductive material to achieve a sufficient level of through-planeconductivity.

Woven carbon cloth can be utilised as ES base material, but it isexpensive and restricts the options to reduce the ES thickness. Metalfibers suitable for fuel cell ES are expensive since they need to beoxidation and corrosion resistant, and therefore must be selected fromthe noble metals such as platinum, iridium, rhodium, or osmium.

Consequently, what is required is a low-cost ES with high porosity aswell as through-plane conductivity which is manufactured using anindustrial scale continuous production process.

SUMMARY OF THE INVENTION

According to the present invention, electrode substrates forelectrochemical cells, more specifically for fuel cells, with highporosity and good electrical conductivity and methods for theirmanufacture are disclosed. The electrode substrates comprise acarbonised or graphitised fiber (also often referred to as “graphitefiber”) web structure with a high electric through-plane conductivity,said web structure being covered and filled with impregnation agent, andoptionally, with chemically inert and conductive particles.

The ES described in this invention are made from conductive preformedweb structures based on graphitised fibers that preferably have athrough-plane conductivity of more than 1 S/cm, more preferably 6 S/cmor more, and especially preferred in excess of 6.4 S/cm. Through-planeconductivity is determined as described in WO 98/27606, which is hereinincorporated by reference.

The ratio of through-plane conductivity to in-plane conductivity of theES according to this invention is usually at least 0.25, preferably morethan 0.42, and especially preferred more than 0.66. In-planeconductivity can be measured by a similar method, wherein two pairs ofcontact blocks are pressed on an ES material, and a current of 3 Ampereis applied between the two pairs of contact blocks. In-planeconductivity is then calculated from the voltage drop between the twopairs of blocks, the applied current, and the cross-section of thesubstrate and the distance between the two pairs of blocks.

The web is characterised by a high fraction of graphitised fibers beingoriented not in planar direction. Graphitised fibers are highlyanisotropic, thus their conductivity along the fiber axis is superior tothe conductivity perpendicular to the fiber axis. Therefore, a highfraction of graphitised fibers with non-planar orientation in a webstructure results in a high through-plane conductivity. Such webstructures comprise, but are not limited to, woven cloth, needled felt,hydroentangled non-woven, and knitted fabric. High fraction in thiscontext means at least 20 percent, preferably, at least 30%, and mostpreferred, more than 40% of all graphitised fibers. Such a web structureis shown in FIG. 3. The graphitised fibers 10 form a web which impartsthe preferential orientation to the fibers.

The current method to manufacture such graphitised fiber based webstructures is to use oxidised polyacrylonitrile (PAN) fibers followed bygraphitisation in batch or continuous furnaces. The utilisation ofcarbon fibers for manufacturing such structures is prevented by the highstiffness of carbon fibers. Even forming such web structures fromoxidised PAN fibers results in low manufacturing speed and relativelyhigh scrap rates because these fibers are also difficult to processbecause of their mechanical properties.

A method to circumvent these problems is the highly efficient productionof such web structures directly from PAN fibers, such as Dolanit®12-based PAN fibers, which are then treated in a continuous oxidationfurnace as described in U.S. Pat. Nos. 3,914,960 and 5,853,429, followedby a graphitisation step. This entire process is very cost effective andyields a uniform continuous material.

The web structures need to be processed further, in order to adjusttheir porous structure, bending stiffness, thickness and other desiredfinal properties.

For this purpose, the web structure is impregnated th a liquid which maycontain chemically inert and electrically conductive particles. Thoseimpregnated conductive web structures are calendered to adjust the finalthickness and the material homogeneity. During this step, the calenderedmaterial is heated and dried.

In another embodiment of this invention, the calendering step isfollowed by a final heat treatment. The conditions of this final heattreatment procedure are determined by the final ES properties. FIG. 4 isa cross-section of such an impregnated web structure according to thisinvention. The graphitised fibers 10 forming the web structure aremainly aligned perpendicular to the planar direction (the horizontalaxis in this figure) and are enclosed by the impregnation agent 12 andoptionally the chemically inert and electrically conductive particles.The pores 13 are still large and their size and shape are adjustableaccording to the requirements of the particular fuel cell electrode.

The process according to the invention yields a roll of low-cost ES withfinal properties superior to the prior art products. Such ES rolls canbe used for subsequent fuel cell electrode manufacturing steps onindustrial scale.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded side view of a single cell showing the structureof a phosphoric acid, or a membrane, fuel cell, viz., a flat plateelectrode fuel cell where the electrode substrate of the presentinvention can be applied.

FIG. 2 is a prior art side view (cross-section) of a carbon fiber ES,where the carbon fibers are mainly aligned in planar direction.

FIG. 3 is a side view (cross-section) of a conductive graphitised fiberweb structure according to this invention.

FIG. 4 is a side view (cross-section) of an ES according to thisinvention.

FIG. 5 is a side view (cross-section) of an ES with a pore size gradientaccording to this invention.

FIG. 6 is a side view (cross-section) of an ES with a property gradientperpendicular to the planar direction according to this invention.

All cross-sections (FIGS. 2 through 6) have the plane of the ES parallelto the horizontal axis.

DETAILED DESCRIPTION

In one embodiment of the web structure, a hydroentangled non-woven madefrom oxidised PAN fibers, such as ®PANOX fibers from SGL CARBON, isemployed.

In another embodiment of this invention, the web structure is formed bya felt needling process employing oxidised PAN fibers.

In a further embodiment of the web structure, a woven cloth based onoxidised PAN fibers is used.

In one embodiment of the ES production process, web structures made fromPAN fibers are treated in a continuous oxidation furnace as described inU.S. Pat. Nos. 3,914,960 and 5,853,429. This process can be referred toas “Direct Oxidation Process” (DOP).

The PAN based web structures are heat treated in an oxidising airatmosphere at 200 to 350° C. under tension. Tension is required toachieve better mechanical properties of the oxidised product as well asto prevent high shrinkage of the material during the DOP.

In the next step according to this invention, either web structure madefrom oxidised PAN fibers or web structures resulting from DOP aregraphitised to achieve a high through-plane conductivity. Thegraphitisation furnace can be a batch furnace, but preferably acontinuous-processing furnace with a graphite muffle is employed. Underinert gas atmosphere, the web structure is treated at temperaturesbetween 1500 to 2500° C., most preferably between 1650 to 2000° C.

Such graphitised web structures as shown in FIG. 3 are characterised byhigh through-plane conductivity of more than 1 S/cm, preferably between6 and 10 S/cm and porosity ranging from 80% to 95%, preferably 85 to90%. The mean pore sizes (diameter) of such web structures can be foundin the range from 75 μm to 500 μm. Porosity is defined here as the ratioof pore volume to total volume, measured in percent.

The graphitised web structures are subsequently impregnated with animpregnation agent or a mixture of an impregnation agent with chemicallyinert and electrically conductive particles.

In one embodiment of this invention, the impregnation agent is a liquidsolution or dispersion of a thermoplastic resin which comprises, but isnot limited to, polyethylene, polypropylene, amorphous thermoplasticcopolymers made from ethylene, propylene or mixtures thereof with cyclicor polycyclic olefins such as norbornene and ethylidene norbornene,polyphenylene sulphide, polystyrene, ABS (thermoplastic acrylonitrilebutadiene styrene terpolymers), styrene maleic anhydride copolymers, andpartially fluorinated resins such as PVDF, ethylene tetrafluoroethylenecopolymers, and the like. In another embodiment this impregnation agentcan be a thermoset resin like phenolic resins, furane resins, or epoxyresins. Most preferred are such resins that have a high mass fraction ofaromatic moieties, such as phenolic resins. A high mass fraction in thiscontext means at least 30 percent, preferably 40 percent, and mostpreferred at least 50 percent of aromatic moieties. Solutions of suchthermoset resins, or solutions of non-cross-linked precursors of suchthermoset resins may also be employed.

In a further embodiment the impregnation agent can be a carbonisablematerial such as, but not limited to, coal tar pitch, petroleum pitch,tall pitch or wood pitch, or a solution thereof in an appropriatesolvent.

Optionally, chemically inert and conductive particles can be added tothe impregnation agent for adjusting the viscosity of the impregnationmixture and for adjusting certain properties of the final ES such as themicroporous structure, in-plane conductivity, contact resistance, andmechanical strength. Among the particles that can be added to theimpregnation agent, mention is made of graphitic materials, such asgraphitised fibers, graphite nanofibers, graphite flakes, graphitepowders, metal carbide materials such as metal carbide powders, metalcarbide fibers, and metal carbide nanofibers, which lists are notexhaustive, and serve for illustration only.

In accordance with the present invention, the amount of impregnationagent, additional particles, and impregnation mixture applied to the webstructure depends on the nature of the used agents and particles, on theimpregnated web structure as well as on the desired properties of thefinal ES. Generally, the ratio of the mass of the impregnation agent orimpregnation mixture to the mass of the web structure may be from 5:100to 400:100, preferably from 10:100 to 300:100, most preferred from25:100 to 250:100.

The impregnation agent or the impregnation mixture can be applied byvarious techniques. Such techniques include, but are not limited to,transfer coating, roller coating, dipping, doctor-blade techniques, andspraying.

In one embodiment of this process, the impregnation agent orimpregnation mixture is applied single-sided only resulting in a porousstructure with a gradient. Such a structure is shown as a cross-sectionin FIG. 5. Single-sided in this context means that the impregnatingagent is applied to the web structure on one side only (the bottom sidein the structure as depicted in FIG. 5), usually resulting in anon-homogeneous saturation of the web with the impregnating agent, withthe region opposite to the side where the impregnating agent had beenadministered has a lower content of impregnating agent, and therefore, ahigher fraction of open or unfilled pores. Such a gradient porestructure facilitates a better reactant distribution to the catalystlayer. It has been found that the pores of such single-side impregnatedstructures have a cone-like shape. This is illustrated in FIG. 5, wherea web of graphitised fibers 10 has been impregnated, the impregnatingagent having been administered from the bottom side of the web only. Theimpregnating agent 12 (or its carbonised residues) is concentrated inthe lower part of the cross-section, leading to formation of small pores15 in the bottom region of the ES, while larger pores 14 are formed inthe upper regions of the ES, where less impregnation agent haspenetrated.

Another embodiment of this process comprises a simultaneous double-sidedimpregnation process by using different impregnation agents orimpregnation mixtures for the opposite (top and bottom) faces of the webstructure. This procedure can be required for imparting differentproperties into the top and bottom faces of the ES such as, but notlimited to, surface roughness, pore size, microporosity, water contactangle, and capilarity. The result of such impregnation method is shownin FIG. 6, where the web of graphitised fibers 10 has regions where onlyimpregnation agent I (administered from the top face) has penetrated andformed a porous layer 16, whereas impregnation agent II which had beenadministered from the bottom face has only penetrated the lower regionand formed another porous layer 17. As in FIG. 5, the individual poresformed have a conical structure, the narrow region being adjacent to theside from where the impregnation agent has been applied.

In a further embodiment of the impregnation process, the top and bottomface of the web structure may be impregnated by two subsequent steps.

If such an ES is cut perpendicularly to the planes, a property gradientperpendicular to the planar direction can be detected which results fromusing different impregnation agents or impregnation mixtures for the topand bottom side of the ES. This fact is illustrated in FIG. 6 (seeabove). The gradient is governed by the diffusion velocity of theimpregnating agents within the porous web structure. If the impregnatingagents also comprise particulate fillers, a filtration effect may beadded if the pore size is not considerably larger than (more than twicethe size of) the filler particles.

Following the impregnation, the web structure is submitted to acalendering step. The compression force is adjusted to the amount andnature of impregnation agent, additional particles, and impregnationmixture and also depends on the impregnated web structure as well as onthe desired properties of the final ES. In general, during thecalendering step the impregnated web structure is subjected to acompression force resulting in a thickness reduction between 2 to 15%,preferably 5 to 10%.

During the calendering step, the web structure is submitted to elevatedtemperatures. If the applied temperatures range between 30 and 250° C.,the web structure is usually heated within the calender itself. Iftemperatures up to 500° C. are required, additional heaters areutilised. Such heaters may be, but are not limited to, IR-heaters,electrical resistance heaters, and hot gas blowers. The temperaturesapplied during the calendering step are preferably selected to dry theimpregnated material, melt or cure the impregnation agent and to keepthe calendered material at the desired final thickness.

Another embodiment of this invention comprises a final heat treatmentstep. This step may be required for achieved certain ES properties. Sucha final heat treatment step can be applied when the impregnation agentis a carbonisable material, such as a thermoset resin or pitch. Thisfinal heat treatment is carried out in a continuous-processing furnacewith a ceramic muffle under nitrogen atmosphere approximately at from500 to 1200° C.

All ES manufacturing processes described in this invention can becarried out in an industrial scale thus providing a low-cost product.Furthermore, ES manufactured by the methods described here have asufficiently low bending stiffness allowing winding and take-up onreels, and further processing as roll-material without compromisingmechanical properties required for such industrial scale processingsteps.

While particular materials, processes and embodiments of this inventionhave been described, this description is not meant to be construed in alimiting sense. It is understood that various modifications of thepreferred embodiments, as well as additional embodiments of theinvention, will be apparent to those skilled in the art upon referenceof this description without departing from the spirit and scope of theinvention, as defined in the following claims. It is thereforecontemplated by the appended claims to cover any such modifications orembodiments that fall within the true spirit and scope of the invention.

LIST OF FIGURES

FIG. 1 exploded side view of a phosphoric acid or membrane fuel cell

FIG. 2 side view (cross-section) of a carbon fiber ES

FIG. 3 side view (cross-section) of a conductive graphitised fiber webstructure

FIG. 4 side view (cross-section) of an impregnated graphitised fiber webstructure

FIG. 5 side view (cross-section) of an impregnated graphitised fiber webstructure with a pore size gradient

FIG. 6 side view (cross-section) of an impregnated graphitised fiber webstructure with a property gradient

LIST OF REFERENCE NUMERALS IN THE FIGURES

1 Fuel Cell Assembly

1′ Electrode Assembly

2 Electrolyte Layer or Membrane

3 Anode

3′ Anode Support Structure

3″ Anode Catalyst Layer

4 Cathode

4′ Cathode Support Structure

4″ Cathode Catalyst Layer

5,6 Separation or Distribution Plate

7,7′ Current Collector Plates

8,9 Grooves

10 Graphitised Fibers

11 Binder

12 Impregnation Agent

13 Pore

14 Large Pore

15 Small Pore

What is claimed is:
 1. An electrode substrate for an electrochemicalcell, said substrate comprising a graphitized fiber web structure withhigh electrical through-plane conductivity of more than 1 S/cm, said webhaving pore sizes between from 75 μm to 500 μm, and said web structurebeing covered and filled with impregnation agent and optionally withchemically inert and conductive particles.
 2. The electrode substrateaccording to claim 1, wherein said web structures are characterised by afraction of at least 20% of graphitised fibers being oriented not inplanar direction.
 3. The electrode substrate of claim 1, wherein theratio of the through-plane conductivity to the in-plane conductivity isat least 0.25.
 4. The electrode substrate according to claim 1, said webhaving a porosity ranging from 80% to 95%.
 5. The electrode substrateaccording to claim 1, wherein said web structure is a woven cloth. 6.The electrode substrate according to claim 1, wherein said web structureis selected from the group consisting of a needled felt, ahydroentangled non-woven, a woven cloth, and a knitted fabric.
 7. Theelectrode substrate according to claim 1, wherein said impregnationagent comprises a solution or dispersion of resins selected from thegroup consisting of thermoplastic resins, partially fluorinated resins,phenolic resins, furane resins, and epoxy resins.
 8. The electrodesubstrate according to claim 1, wherein said impregnation agentcomprises pitch.
 9. The electrode substrate according to claim 1,wherein said with chemically inert and conductive particles comprisegraphitic materials.
 10. The electrode substrate according to claim 1,wherein said with chemically inert and conductive particles comprisemetal carbide materials.
 11. A process for producing an electrodesubstrate according to claim 1, said process comprising graphitizationof a fiber web structure based on oxidized polyacrylonitrile fibersunder inert gas atmosphere at temperatures between 1500 to 2500° C. 12.A process for producing an electrode substrate according to claim 1,said process comprising treating a web structure made from oxidizedpolyacrylonitrile fibers in an oxidizing air atmosphere at 200 to 350°C. under tension followed by graphitization under inert gas atmospherebetween 1500 to 2500° C.
 13. A process for producing an electrodesubstrate according to claim 1, said process comprising impregnation andcalendering of a graphitised fiber web structure.
 14. The process ofclaim 13 wherein the ratio of the mass of the impregnation agent orimpregnation mixture to the mass of the web structure is from 5:100 to400:100.
 15. The process of claim 13 wherein the impregnation agent orimpregnation mixture is applied single-sided resulting in a porousstructure with a gradient.
 16. The process of claim 13 wherein asimultaneous double-sided impregnation process is applied by usingdifferent impregnation agents or impregnation mixtures for the top andbottom face of the web structure.
 17. The process of claim 13 whereinthe top and bottom face of the web structure are impregnated by twosubsequent steps.
 18. The process of claim 13 wherein the calenderingstep results in a thickness reduction of the impregnated web structureof between 2 and 15%.
 19. The process of claim 13 wherein theimpregnated web structure is submitted to elevated temperatures duringthe calendering step.
 20. The process of claim 13 wherein theimpregnated web structure is heated by the calender itself in thetemperature range between 30 to 250° C.
 21. The process of claim 13wherein the impregnated web structure is heated by external heaters, andtemperatures from 250 to 500° C. are applied.
 22. A process forproducing an electrode substrate according to claim 1, said processcomprising impregnation and calendering of a graphitised fiber webstructure and an additional final heat treatment step.
 23. The processof claim 22 wherein the final heat treatment step is carried out in acontinuous-processing furnace with a ceramic muffle under nitrogenatmosphere at 500 to 1200° C.
 24. An electrode substrate for anelectrochemical cell, said substrate comprising a graphitized fiber webstructure with high electrical through-plane conductivity of more than 1S/cm, said web structure being covered and filled with impregnationagent and optionally with chemically inert and conductive particles, andwherein said web structure is selected from the group consisting of aneedled felt, a hydroentangled non-woven, and a knitted fabric.
 25. Theelectrode substrate according to claim 24, wherein said impregnationagent comprises a solution or dispersion of resins selected from thegroup consisting of thermoplastic resins, partially fluorinated resins,phenolic resins, furane resins, and epoxy resins.
 26. The electrodesubstrate of claim 24, wherein said web structures include a fraction ofat least 20% of graphitized fibers being oriented not in planardirection.
 27. The electrode substrate of claim 24, wherein saidimpregnation agent comprise pitch.
 28. The electrode substrate of claim24, wherein said chemically inert and conductive particles comprisegraphitic materials.
 29. The electrode substrate of claim 24, whereinsaid chemically inert and conductive particles comprise metal carbidematerials.
 30. A process for producing an electrode substrate accordingto claim 24, said process comprising graphitization of a fiber webstructure based on oxidized polyacrylonitrile fibers under inert gasatmosphere at temperatures between 1500 to 2500° C.
 31. A process forproducing an electrode substrate according to claim 24, said processcomprising treating a web structure made from polyacrylonitrile fibersin an oxidizing air atmosphere at 200 to 350° C. under tension followedby graphitization under inert gas atmosphere between 1500 to 2500° C.32. A process for producing an electrode substrate according to claim24, said process comprising impregnation and calendering of agraphitized fiber web structure.
 33. The process of claim 32 wherein theratio of the mass of the impregnation agent or impregnation mixture tothe mass of the web structure is from 5:100 to 400:100.
 34. The processof claim 32 wherein the impregnation agent or impregnation mixture isapplied single-sided resulting in a porous structure with a gradient.35. The process of claim 32 wherein a simultaneous double-sidedimpregnation process is applied by using different impregnation agentsor impregnation mixtures for top and bottom faces of the web structure.36. The process of claim 32 wherein the top and bottom faces of the webstructure are impregnated by two subsequent steps.
 37. The process ofclaim 32 wherein the calendering step results in a thickness reductionof the impregnated web structure of between 2 and 15%.
 38. The processof claim 32 wherein the impregnated web structure is submitted toelevated temperatures during the calendering step.
 39. The process ofclaim 32 wherein the impregnated web structure is heated by the calenderitself in the temperature range between 30 to 250° C.
 40. The process ofclaim 32 wherein the impregnated web structure is heated by externalheaters, and temperatures from 250 to 500° C. are applied.
 41. A processfor producing an electrode substrate according to claim 7 whichcomprises impregnation and calendering of a graphitized fiber webstructure and an additional final heat treatment step.
 42. The processof claim 41 wherein the final heat treatment step is carried out in acontinuous-processing furnace with a ceramic muffle under nitrogenatmosphere at 500